The structure of music gets very complex very quickly when one is studying it in depth. It is not simply a matter of conceptual difficulty, which is some instances can be overcome fairly quickly, but it is the compounding of the conceptual problems with that of memory, i.e., remembering the relationships of different key signatures to one another and the different chord types to one another, which chord types of course can be fit into any of the key signatures. There is the additional problem of recognizing chords and their different inversions, and on top of all of this is the task of developing the manipulative skill necessary to apply one's knowledge to a keyboard or other musical instrument.
For these reasons it is very helpful to students as well as accomplished pianists to have training aids which are able to graphically display and integrate a number of different musical facts in a fairly simple format, and which by virtue of the physical movement of the component parts define physically analogies to the mental structure representing music.